The present invention is directed to the preparation of composite printing elements and, more particularly, to the direct transfer of digital images to such composites without the use of phototools.
Relief image printing plates are used in both flexographic and letterpress processes for printing on a variety of substrates, including paper, corrugated stock, film, foil, and laminates. The photocurable elements that are used to make relief plates typically include a support layer and one or more layers of photocurable polymer in the form of solid sheets. The printer typically peels a cover sheet from the element to expose the photocurable polymer and places a silver halide photographic negative or some other masking device upon the photopolymer. The negative-bearing photocurable element then is exposed to ultraviolet (UV) light through the negative, thereby causing exposed areas of the element to harden, or cure. After the uncured areas of the element are removed, cured polymer remains as the relief printing surface.
Corrugated boxes and other, relatively large objects such as, for example, flexible packaging materials are typically printed using relief image printing plates that bear actual printing on only a small portion of their total surface area. One way to print such an object is to prepare a single relief image plate having a surface area corresponding to the total surface area of the object. Since only a portion of the object""s surface needs to be printed, however, only a portion of the relief image plate will actually be used for ink transfer. The remainder of the plate will be unused and, essentially, wasted.
To minimize such waste, those skilled in the art often print relatively large objects with composite printing elements that are prepared by mounting a plurality of relief image printing elements on a common printing cylinder, sleeve, or carrier sheet. The individual elements, however, are mounted only on those portions of the carrier that correspond to the portions of the object that actually need to be printed. Although such composite elements do minimize waste, the current system for mounting their constituent relief image elements is laborious and requires careful adhesion of the elements to the carrier while assuring registration to within 0.005 inches on-press for high quality printing and multi-color reproduction. For multi-color reproduction, wherein a single element is used for printing each of the individual colors, accurate registration of the plates with respect to one another is crucial.
When a flexographic photopolymer printing element is mounted on a printing cylinder the printing surface of flexographic photopolymer printing element is stretched more than the back surface, i.e., the surface closest to the cylinder. This elongation of the printing surface typically causes distortion of the printed copy unless the elongation is compensated for during the manufacturing of the printing element. To compensate for the elongation of the printing surface, the printer must perform a tedious process of determining the precise percentage reduction of the negative image on the printing element so that, when mounted and stretched, a true image can be printed. The success of this process depends upon the printer""s calculation of two factors that effect the image elongationxe2x80x94the thickness of the photopolymer layer above the neutral axis and the circumference of the plate cylinder (including the thickness of the adhesive or other material used to mount the element to the. cylinder. Inaccurate measurements can result in a distorted image.
Consequently, there remains a need in the art for alternative processes for preparing composite printing elements. In particular, there remains a need for alternative processes for accurate registration of the constituent relief image printing elements on arcuate surfaces without the risk of image elongation.
The present invention provides methods for preparing composite printing elements without the need for individual registration of constituent relief image elements and, preferably, without the need for compensating for image elongation due to cylindrical mounting. These methods comprise the steps of transferring registration information to an outer face of a printing element, disposing a photocurable element upon the surface of the printing element in approximate register, and then transferring a computer-generated negative image onto the photocurable element. In preferred embodiments, the printing element has a substantially arcuate cross-section.
Approximate registration of the photocurable elements can be achieved by transferring computer-generated registration information to a surface of the printing element. This can be achieved, for example, by transferring some visually perceptible material (such as ink from an ink jet print head) onto the printing element, or by scoring or otherwise deforming the element. The registration information can, for example, comprise a series of images whose respective shapes correspond to the outlines of the individual photocurable elements.
In one preferred embodiment of the present invention, transfer of the computer-generated negative image to composite elements of the invention preferably is achieved by ejecting a negative-forming ink from an ink jet print head. The ink preferably is substantially opaque to actinic radiation in at least one wavelength region effective to cure photocurable material within the element and substantially resistant to polymerization upon exposure to actinic radiation in the wavelength region. Following the negative transfer step, the ink-bearing element can be exposed to actinic radiation in the wavelength region for a time and under conditions effective to cure exposed areas of the photocurable material, and unexposed (i.e., uncured) areas then are removed to provide the relief printing surface.
In another preferred embodiment, transfer of the computer-generated negative image to composite elements of the invention is achieved by using a laser to ablate a photoablative mask layer on the photocurable elements"" respective surfaces. The laser is employed to selectively ablate, or remove, the photoablative mask layer such that the areas where the photoablative mask layer is ablated will cure, or harden, upon exposure to the UV light and the areas where the photoablative mask layer is not ablated will remain uncured. Following front exposure of the exposed areas of the photopolymer, uncured photopolymer, i.e., the photopolymer under the areas of the photoablative layer that are not laser-ablated, is removed from the mounted photocurable elements.